1. Field of the Invention
The present invention relates to a blood syringe, and more particularly to a blood syringe with a ratchet device to prevent the piston from detaching from the plunger while drawing blood.
2. Description of Related Art
As shown in FIG. 5, a conventional blood syringe in accordance with the prior art comprises a hollow barrel (50) with a connecting block (51), a plunger (60) with a piston (61) and a cap (70). The plunger (60) is received in the hollow barrel (50) and partially extends through the hollow barrel (50). The cap (70) closes the end of the connecting block (51) on the hollow barrel (50) opposite to the plunger (60).
An external thread (501) is formed on one end of the hollow barrel (50) and an internal circular flange (502) is formed inside the other end. The connecting block (51) has an open end and a closed end. The inside of the open end of the connecting block (51) is threaded and screws onto the external thread (501) on the hollow barrel (50). The closed end of the connecting block (51) includes a hollow needle hub (512) extending out from the center of the closed end and a flange (511) extending longitudinally out from the closed end around the needle hub (512). A through hole (513) is defined in the end of the needle hub (512). The through hole (513) communicates with the open end of the connecting block (51).
A male thread (602) is formed on one end of the plunger (60) and a button (603) is formed on the other end. A neck (601) is formed between the male thread (602) and the plunger (60). The plunger (60) is broken off at the neck (601) after drawing blood has been completed. The piston (61) has a hollow stub (611) extending from one end with an internal female thread (not numbered).
The cap (70) includes an open end to snugly fit over the flange (511) of the connecting block (51) and a closed end having a prong (71) extending inward. The prong (71) extends through the through hole (513) of the needle hub (512) when the cap (70) is attached to the connecting block (51).
However, carelessness by the technician while drawing blood may cause the piston (61) to detach from the plunger (60), and the blood in the hollow barrel (50) will flow back into the examinee's body. Also, since the cap (70) is simply pressed onto the connecting block (51), the cap (70) can easily be detached from the connecting block (51) if bumped.
The present invention has arisen to mitigate and/or obviate the disadvantage of the conventional blood syringe.